The Electrochemical Formation of Columnarstructured Gold Surfaces

Representative 1000x1000 nm2 ex-situ STM images of rough gold films of different average thicknesses (hs) show the development of a typical columnar structure (Fig.l(a)-(c)) [18]. These films are produced by electroreducing, at 100 nm s"1 in 0.5 M H2SO4 at 298 K, gold oxide layers formed on gold plates by anodization at 2.4 V (vs a saturated hydrogen reference electrode) in the same acid solution. Columnar-structured gold film surfaces exhibit a self-affine fractal behavior as the result of the type of aggregation process involved in the oxide layer electroreduction [19].

Fig. 1. Constant current (1000x1000 nm2) 3D STM images of gold elqflrodeposits grown at 100 nm s"1 on gold at 7= 298 K. (a) hs (t) = 35 nm; (b) hs (t) = 175 nm; (c) hs (t) = 318 nm.

Data resulting from STM imaging were plotted as log OhM vs. log <Ls> (Fig.2(a)) and log cohM vs. log hs (t) (Fig.2(b)) to derive the value of exponents a and J3 included in Eqs. (2) and (3). These plots exhibit two sets of exponents, each one of them being valid within a certain scale length Ls, i.e., the microscope imaging resolution. Those plots also define a crossover point at a certain critical distance which coincides with ds, the average size of the column cross-section. Thus, when Ls < ds, it results in a(l) =

0.90±0.06 and /5(I) = 0.31±0.06, whereas for Ls>ds, it yields a(tt) = 0.49±0.05 and (3(H) = 0.51±0.08.

Fig. 1. Constant current (1000x1000 nm2) 3D STM images of gold elqflrodeposits grown at 100 nm s"1 on gold at 7= 298 K. (a) hs (t) = 35 nm; (b) hs (t) = 175 nm; (c) hs (t) = 318 nm.

It should be noted that for is < d%, the value of a is close to those predicted by atomistic models incorporating surface diffusion, and a continuum model described by

Fig. 2. (a) Log (»„M vs log Ls plot. Gold electrodeposits with hs (t) = 886 nm. (b) log a>M vs log hs (i) plot, (o) Ls = 35 nm; (A) Ls = 398 nm.

Eq. (8). The value of ¡3 is, however, slightly greater than the predictions of these models. This difference is presumably caused by nonlocal effects produced by the electric field operating at the growing interface [20]. Likewise, the difference in the value of/?leads to z « 3, and a coarsening exponent l/z « 0.33.

The set of exponents obtained for Ls > involve crystal-crystal interaction terms which characterize solid grained-structures. These terms are not considered in the physical background representing any of the available growth models. Theoretical work is in progress to provide a deeper understanding of these exponents [21]. Nevertheless, the main conclusions derived from these results are the existence of two distinguishable domains of roughness at columnar-structured metal electrodes, and the dominant participation of surface atom diffusion in the smoothening of the columnar surface.

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